Growing Plants with Artificial Light
In the preceding pages: Use of Artificial Light in the Greenhouse, and The Effects of Light on Plant Growth, we have shown that both the amount and the colour of light, influence plant growth and development. We have also discussed various methods of installing lamps.
Now we explain the techniques used for growing popular greenhouse plants under artificial light.
Many plants raised in greenhouses can benefit from extended day lighting and/or supplementary lighting. Supplementary lighting is the use of artificial light to supplement poor natural light during winter months. The object is to encourage plants to start into growth sooner and come into crop or flower earlier.
Night-break lighting provides an additional period of light in the hours of darkness. This often proves an economical way of lengthening the plant’s day hours. Times given for night-break lighting are centred around 1.00 am.
The following paragraphs discuss a wide range of plants, from tomatoes to antirrhinums.
Tomatoes need a minimum night temperature of 13°C (55°F). They also need plenty of light, and in the early months of the year natural light is not strong enough to produce sturdy, well-developed plants. So under normal cool house conditions of 7°C (45°F) it is not possible to start picking until mid summer (June). By then shop prices will have fallen and the effort and expense in producing the crop may seem hardly worthwhile. The ideal, therefore, is to use artificial light to start them cropping earlier.
Sow the seeds in pans in early spring or even late winter (February or January), and germinate in a temperature of 21°C (70°F) — an electric propagator provides the right conditions. As soon as the seeds are large enough to handle, prick out into 9cm (3-1/2 in) pots and grow on under artificial light. The total average illuminance should be 10,000 lx so, as only about 12 plants will be needed in the average gardener’s greenhouse, a 400 watt MBFR/U lamp mounted 970mm (3 ft 2 in) above the plants should be adequate. Two 1500mm (5 ft), 65 watt warm white fluorescent tubes, each of them mounted 380mm (15 in) above the plants, would also be effective. Three weeks’ irradiation can be sufficient; during mid spring (March) the lamps can be turned off on clear sunny days. Tomato plants should never be irradiated longer than 16 hours each day or growth and flowering may be retarded. You can start before early spring (mid February) but you would then have to maintain a continued minimum night temperature of 13°C (55°F) after the propagating period.
Aubergines and sweet peppers These vegetables can be grown out of doors during a warm summer but cropping may be disappointing. For better results start them early, perhaps in a propagator in early spring (February), and treat them as you would tomatoes. You can cut your running costs by irradiating the young plants at the same time as the tomatoes.
Lettuce is a major commercial green- house crop during the winter months, but under normal light conditions it may take six weeks or more before the young plants are ready for planting out in the greenhouse border or heated frame. To speed things up sow pelleted seed individually in 5cm (2 in) pots with a peat-based seed compost and germinate them in a temperature of 10°C (50°F). If you irradiate the seedlings 24 hours a day for 14 days immediately after, planting out time can be advanced by up to four weeks. In this way you can grow and clear a lettuce crop before the border is needed for tomatoes. An illuminance of 5000 lx is adequate. If, however, you have problems in blacking out your greenhouse at night so as to avoid disturbing neighbours, irradiate for 12 hours daily at 10,000 lx.
Daffodils and other narcissi, hyacinths, tulips and crocuses can be forced successfully under 40 watt general service lamps installed on a frame on the basis of 2.5 lamps per square metre or yard.
As the foliage and flower-stalks increase in height so the frame is raised accordingly. Completely darkened sheds and cellars are adequate for forcing as long as you can maintain a temperature of not less than 16°C (61°F). Store the bulbs in boxes in mid to late autumn (late September) and stand them in a cool place outdoors, such as by the north side of the house or wall, and covered with 15cm (6 in) of straw or 10cm (4 in) of moist peat. Make sure the straw and peat are kept moist. Towards mid winter (end of November) when the shoots are about 6.5cm (2-½ in) long the bulbs will be ready for forcing.
Daffodils and other narcissi need 16°C (61°F) and if pre-cooled bulbs (bulbs specially treated for forcing) are used. Successive batches can, with a careful selection of varieties, be forced to provide a continuous supply from mid winter to late spring (December to April). For tulips 18°C (64°F) should be the aim until buds are showing, then drop to 16°C (61°F) until flowering. Keep hyacinths in the dark for the first 1-7 days depending on the variety. Temperatures need to be higher than for narcissi or tulips; start off with 24°C (75°F) and then, when colour shows, drop to 18°C (64°F). Hyacinths need a relatively high humidity, but still follow the general watering rule for all bulb flower crops — restrict it to just enough to ensure good, firm growth.
If you have space under the greenhouse bench, then you can force them there, provided you exclude daylight with black polyethylene sheeting. Keep up temperature by means of a small fan heater if necessary, and pay careful attention to ventilation. The daily lighting period can be restricted to 12 hours and this can be done at night to economize on heating.
Although considerable success has been achieved in producing good quality blooms during the winter months, those being produced between late winter and mid spring (January and March) were inferior to those flowering in or before mid winter (December). As a result of research, growers are now using supplementary lighting as well as the normal night-break techniques, to maintain bloom quality throughout the winter.
For the purpose of all-year-round production, chrysanthemum varieties are divided into response groups: for example a 10-week variety is one that requires only 10 weeks from commencement of short days (when night-break lighting is finished) to flowering. The night-break lighting is used between early autumn and late spring (August and April) to prevent premature flower-bud development and encourage vegetative growth, and varies from two hours each night in early autumn (August) to five hours in mid winter (December). As the first two weeks of short days are critical, so far as quantity of daylight is concerned, it is necessary to supplement this with artificial light between the beginning of winter (November) and the end of early spring (February).
For this purpose 400 watt MBFR/u or warm white fluorescent lamps are used to produce 7200 lx, which will be enough to give gratifying results.
Saintpaulia (African violet)
So long as temperatures can be maintained at 18-21°C (64-70°F) these plants will grow normally under the greenhouse bench. Two 1500mm (5 ft), 80 watt warm white fluorescent lamps 380mm (15 in) above the plants will be adequate for a space 760mm (2-½ ft) wide and 1400mm (4-½ ft) long. The daily lighting period should be 12-14 hours.
Special greenhouse forcing varieties should be used. Sow between late autumn and early winter (October and early November). Plants are grown in a temperature of 10°C (50°F) and high temperatures should be avoided. Make quite sure you remove all sideshoots. In a propagating case or under mist with a bottom heat of 21-24°C (70-75°F) germination should take place after three days. Five days later the seedlings can be placed under a 400 watt MBFR/U lamp or warm white fluorescent tubes from 1700 hours to 0700 hours. This lighting should be switched off after about two weeks. This period of long days will enable you to have an early show of flowers.
The Gardener’s Seasons
early spring (February)
mid spring (March)
late spring (April)
early summer (May)
mid summer (June)
late summer (July)
early autumn (August)
mid autumn (September)
late autumn (October)
early winter (November)
mid winter (December)
late winter (January)